A "miracle fat burner!" "One of the most important discoveries made" in weight loss science, the heart surgeon Dr. Mehmet Oz said about the little pills — which are produced by grinding up raw, unroasted coffee, and then soaking the result in alcohol to pull out the antioxidants.

But alas, the history of dieting is littered with failed concoctions and potions. And now, a study in mice casts doubts on green coffee's weight-loss benefits — and even offers some preliminary evidence that it could be harmful.

The main ingredient in green coffee extract — an antioxidant called chlorogenic acid — didn't help obese mice shed the pounds over a 12-week period, scientists at the University of Western Australia reported in the Journal of Agricultural and Food Chemistry. Instead, the compound gave the little rodents the early symptoms of diabetes: The animals were less sensitive to insulin and had higher blood-sugar levels between meals, compared with their overweight comrades who didn't get the antioxidant.

Of course, mice aren't people. And such experiments don't prove that green coffee extract isn't safe. But even in people, the evidence that the supplement melts off pounds is, well, slim.

A meta-analysis a few years ago combined the results from three small, short-term trials. The authors found that green coffee extract was associated with losing about 5 pounds. But this slimming effect vanished when the authors analyzed the two studies that used the type of supplement recommended by Dr. Oz — green coffee extract enriched with chlorogenic acid.

More recently, Oz himself jumped into the research realm and offered his own evidence that green coffee isn't a fraud, as he puts it.

Oz had about 100 women from his audience run an experiment. He sent half of them pills with green coffee extract and the other half placebos. "We found that taking green coffee extract doubles your weight loss," Oz said on his TV show in September.

Sounds fantastic. But let's take a closer look at the study. It lasted two weeks. And on average, the women who took the coffee extract dropped 2 pounds, while those who got the placebo lost an average of 1 pound. Was the difference statistically significant? We don't know. Oz hasn't published the experiment, and his people didn't respond to our request for comment.

But here's the kicker: Both groups also kept journals recording their diets. And there's strong evidence that keeping track of your diet does improve weight loss.

And a journal is cheaper than green coffee pills. Even Oz admits that. "I know $30 a day costs a lot," he said on his show. "If that's too much for you, the free option is a food journal."